Joel
I recently upgraded to 10 and no problems so far, for some context, I use PD for Windows based software development and have around 6 VM's of different types (Win7, 8, Server 2008, Server 2012 etc) all with vanilla guest O/S configurations.
Before upgrading I backup the VM's shut them down and remove all snapshots, the VM configs are all set to 1 CPU and max 10GB RAM - sounds counter intuitive but you get much better performance this way than maxing out the VM RAM/CPU spec.
Once a VM is started, let the VM Parallels Tools upgrade itself and then restart.
Before starting a VM with PD10 for the first time, you may want to consider turning off realtime disk optimisation and reviewing the VM config because there are new configuration options.